1. Technical Field
The present invention relates to improvements of a piezoelectric resonator element and a piezoelectric device housing a piezoelectric resonator element in a package or case.
2. Related Art
Piezoelectric devices, such as piezoelectric resonators and piezoelectric oscillators, have been widely used in small information apparatuses typified by HDDs (hard disk drives), mobile computers and IC cards, mobile communication apparatuses typified by cellular phones, car phones and paging systems, piezoelectric gyro sensors, and so on.
FIG. 12 is a schematic plan view illustrating one example of piezoelectric resonator elements that have been typically used for piezoelectric devices. FIG. 13 is an end view cut along the line A-A of FIG. 12.
Referring to the drawings, the outer shape of a piezoelectric resonator element 1 as a tuning fork type one is formed by etching a piezoelectric material such as quartz. The piezoelectric resonator element 1 includes a rectangular base portion 2 attached to a package (not shown in the drawing) or the like, and a pair of resonating arms 3 and 4 that are extended from the base portion 2 toward the right of the drawing. Longitudinal grooves 3a and 4a and necessary electrodes for driving are formed on the major surfaces (front and back surfaces) of these resonating arms. An example of documents describing the piezoelectric resonator element is JP-A-2002-261575.
In such a piezoelectric resonator element 1, upon application of a driving voltage thereto via the electrodes for driving, flexural vibration arises so that tips of the resonating arms 3 and 4 come close to and move away from each other. Thus, a signal of a certain frequency is extracted.
It has been desired for such a piezoelectric resonator element 1 to have a smaller size along with miniaturization of the above-described various products provided with a piezoelectric device utilizing the piezoelectric resonator element 1. Therefore, the piezoelectric resonator element 1 must be formed to have as small a size as possible, and it is desired for the total length AL1 of the piezoelectric resonator element 1 to be shortened in particular. In addition, since the miniaturization of the products has uninterruptedly progressed, a structure for allowing a smaller size is needed for the piezoelectric resonator element 1.
Here, the frequency f of the piezoelectric resonator element 1, which is a tuning fork type piezoelectric resonator element like that shown in the drawing, is proportional to W/(l×l) if the length and arm width of the resonating arms 3 and 4 are defined as l and W, respectively.
This means that, if the piezoelectric resonator element 1, which is elongated along one direction, is to be miniaturized and thus the magnitude of the total length AL1 in FIG. 12 is to be decreased, shortening the length l of the resonating arms leads to an increase of the frequency. Furthermore, if the width W of the resonating arms is decreased, the frequency is lowered. Thus, in order to achieve miniaturization while maintaining the frequency, the arm width W must be decreased while the length of the resonating arms is shortened to some extent.
In miniaturization of the piezoelectric resonator element 1, in order to maintain, for example, a frequency of 32 kHz (32.768 kHz), which is a typical frequency, it is required that the length l of the resonating arms 3 and 4 is shortened and the arm width W is decreased. However, in processing the small piezoelectric resonator element 1, if the piezoelectric resonator element 1 is to be processed to have the small arm width w in particular while maintaining the characteristics of the element 1, the following difficulties arise.
Specifically, the difficulties result from a need to process longitudinal grooves 3a and 4a like those shown in FIG. 13 in the resonating arms 3 and 4. Referring to FIG. 13, the thickness t is difficult to change since it is restricted by conditions of a material, such as a quartz wafer, to be processed. Therefore, if the thickness t of a typical resonating arm is 100 μm for example, the thickness t of a miniaturized resonating arm should also be 100 μm.
In contrast to this, as for the arm width W, a miniaturized resonating arm may be required to have a width of about 50 μm although the arm width W of a typical resonating arm is 100 μm. For example, when the arm width is 100 μm, the groove width C1 is about 70 μm and the sidewall width S1 is about 15 μm. However, if the arm width W is about 50 μm, the groove width C1 must be reduced to about 40 μm and the sidewall width S1 must be reduced to about 5 μm, for example.
If such a piezoelectric resonator element is fabricated, the rigidity of the resonating arms 3 and 4 is significantly low. Therefore, in the above-described flexural vibration caused by application of a driving voltage, the amplitude in the Z direction in FIG. 13 is added, and thus the flexural vibration of the resonating arms 3 and 4 in the X-direction turns to flexural vibration shown by arrows SF with exaggeration.
FIG. 14 is a graph illustrating the drive characteristic of miniaturized piezoelectric resonator elements having a typical structure. As the level of a driving voltage is gradually increased along the abscissa of the graph, the frequency variation on the ordinate arises toward the negative direction. This shows that components of the Z-direction vibration of FIG. 13 increase and thus the energy loss of the vibration increases, which is a factor of an increase of the crystal impedance (CI) value.
As effective measures for suppressing the CI value, there is a method in which the longitudinal grooves 3a and 4a described for FIG. 12 are elongated and the formation area of the electrodes for driving is increased. However, a piezoelectric resonator element has plural vibration modes. The frequency of a typically used fundamental wave is 32.768 kHz for example. In contrast to this, the frequency of the second harmonic wave of the piezoelectric resonator element 1 is near 250 kHz. Elongating the longitudinal grooves 3a and 4a can lower the CI value of the fundamental wave. However, the CI value of the second harmonic wave is also lowered. Therefore, if a typical structure is used, many products suffer disadvantages that the CI value ratio, which is the ratio of the CI value of a harmonic wave to the CI value of the fundamental wave, is smaller than 1 as shown in FIG. 15 and thus oscillation easily arises not with the fundamental wave but with the second harmonic wave.